Human brain from above showing the two cerebral hemispheres. The arachnoid membrane and the underlying blood vessels have been left intact over the left cerebral hemisphere but the right hemisphere is exposed. In life, the arachnoid membrane is raised above the brain surface by the cerebrospinal fluid. The brain contains more than 300 billion neurons and is composed mainly of gray matter which originate and process nerve impulses and white matter which transmit the impulses.